A large number of processes for converting hydrocarbons give olefins or olefin mixtures which still contain sulfur impurities. Examples of such processes are the pyrolysis processes such as steam-cracking, visbreaking, coking or catalytic cracking processes.
The so-produced olefins are often suitable for chemical uses but they may also be added to fuel or motor-fuel cuts. For these uses, the presence of olefins results in a number of disadvantages; for example, in motor-fuels, whereas olefins have a high Research octane number (F1), their Motor octane number (F2) on the contrary, is rather poor and requires, at least a partial hydrogenation of the concerned olefins to be improved.
The present generalized manufacture of lead-free gasoline requires the production of motor-fuels of higher octane number. One way of increasing the octane number is to react olefins with an alcohol such, for example, as methanol, to produce the corresponding methyl-alkyl ethers. After etherification, the obtained cut generally contains, in addition to methyl-alkyl ethers, non-etherifiable olefins or olefins only etherifiable to a small extent and also sulfur compounds. The Research octane number of said cut is generally excellent but its Motor octane number is often too low. It may however be improved by partial hydrogenation of the olefins contained in the cut.
In the production of ethers for chemical and pharmaceutical uses, most of the processes use, as the starting material, the corresponding olefins, and the ethers obtained generally contain a small amount of unreacted olefins. The specifications in these uses prescribe a very thorough removal of these olefins so as to comply with very severe tests such as DENIGES test, for example, which requires an olefin content of the ether lower than 20 ppm. Accordingly, this requires a very extensive hydrogenation and hence a very high activity of the catalyst, which must be maintained even in the presence of traces of sulfur compounds.
The olefin hydrogenation techniques are already known and the metals from group VIII are excellent catalysts therefor. But, in the presence of sulfur compounds, it is observed that all these metals are deactivated and (or) poisoned more or less quickly. It is also possible to use metals from group VIII for purifying ethers by hydrogenation of the olefinic impurities. A French patent (No. 1,560,586) discloses such a technique where a nickel or cobalt catalyst is used. When using said technique, the hydrogenation is very effective in the absence of sulfur compounds but, to the contrary with a concentration of sulfur compounds of a few ppm, a more or less quick deactivation of the catalyst is observed. If the temperature is increased in order to compensate for this deactivation, the activity is increased to a certain extent but insufficiently and at the cost of a beginning degradation of the ether by hydrogenolysis; these disadvantages obviously make such a process completely ineffective.